Zero voltage switch means for control of electric load circuit

ABSTRACT

Four forms of the invention are shown and each comprises a zero voltage switching means for an AC electric load circuit having gate controlled semiconductors which are triggered in response to voltages in a condition sensitive bridge network connected in the AC power source for the load circuit. The network is shunted out of the power source circuit upon changes in power source voltage from zero in power source half cycles applied to the network whereby the components of the network are subjected only to relatively low voltages and during a brief portion of the half cycle. In one form of the invention, a resistance in the bridge network is heated in response to energization of the load circuit and tends to cause cycling of the switching means for accurate control of the load.

United States Patent 1 Lorenz 1 Jan. 2, 1973 [54] ZERO VOLTAGE SWITCHMEANS FOR CONTROL OF ELECTRIC LOAD CIRCUIT [52] US. Cl. ..307/252 B,219/494, 219/499,

219/501, 307/252 UA, 307/252 W [51] Int. Cl. ..H03k 17/56 [58] Field ofSearch..307/252 B, 252 H, 252 UA, 252

[56] References Cited .UNITED STATES PATENTS 3,564,205 2/1971 Tyler..219/50l X 3,469,177 9/1969 Lorenz ..2l9/501 X 4/1969 Vesper et al..2l9/50l X 11/1969 Cootey et al. ..2l9/501 X [5 7] ABSTRACT Four formsof the invention are shown and each comprises a zero voltage switchingmeans for an AC electric load circuit having gate controlledsemiconductors which are triggered in response to voltagesin a conditionsensitive bridge network connected in the AC power source for the loadcircuit. The network is shunted out of the power source circuit uponchanges in power source voltage from zero in power source half cyclesapplied to the network whereby the components of the network aresubjected only to relatively low voltages and during a brief portion ofthe half cycle. In one form of the invention, a resistance in the bridgenetwork is heated in response to energization of the load circuit andtends to cause cycling of the switching means for accurate control ofthe load.

6 Claims, 4 Drawing Figures P'A'TENTEDJAM 2191s 3.708.696

sum 1 or 3 ATTORNEYS PATENTEDJAN 2 I873 SHEET 2 BF 1 k i 203v T212 FIG?)INVENTOR.

. JEROME L. LORENZ Maw ATTORNEYS PATENTED 2 1975 SHEET 3 [IF 3 mvswfda-JEROME LORENZ QAA M ATTORNEYS ZERO VOLTAGE SWITCH MEANS FOR CONTROL OFELECTRIC LOAD CIRCUIT BACKGROUND OF THE INVENTION the positive halfcycle to the negative half cycle and w vice versa, and the switchingmeans may be rendered conductive as the power voltage level swings fromzero by a triggering circuit operative to apply a voltage to the gatecapable of firing the switching means. Power control means of the typementioned are referred to as zero voltage switching means. Examples ofthe zero voltage switching means and control circuits therefor referredto are disclosed in US. Pat. Nos. 3,335,291 and 3,486,042. Generally,the triggering circuits were supplied with a DC source which requiredthe use of relatively expensive and unreliable components, such aselectrolytic capacitors. Furthermore, where the triggering circuitsutilized thermistors, the self-heating characteristics of such devicesrendered them inaccurate for temperature controllers. If the sizes ofthe thermistors were increased to reduce the self-heating effect, themass of the thermistor would cause sluggish response to temperaturechanges.

THE PRESENT INVENTION The present invention is an improvement on zerovoltage switching means of the type described in which the triggeringcircuit derives its power from the AC power source for the load, andcomprises means for limiting the voltage and current to the triggeringcircuit during each AC cycle to which the network is subjected. Thecomponents of the bridge network therefore are subject to but a smallfraction of the power source voltage and a minimum of current flow. Theswitching means in the load circuit may or may not be renderedconductive in response to the initial current flow inthe bridge networkcircuit depending upon the condition monitored by the network. Theimproved circuitry has the advantage of permitting a condition' sensingbridge network to receive its power directly from the AC power sourceand yet be comprised of condition sensing elements, such as thermistors,which are of slight mass so that they respond rapidly to temperaturechanges and are not heated by current passed therethrough. By limitingcurrent flow in the triggering circuit, self-heating of components isavoided and therefore greater accuracy may be achieved in the control ofthe switching means. The invention further permits the use of relativelyfew, low cost components without sacrificing reliability.

. Other objects and advantages of the invention will be apparent fromthe accompanying drawings showing preferred forms of the invention andin which:

FIG. 1 is a wiring diagram of an electric heating system for a room orthe like and a control circuit for the system according to theinvention;

FIGS. 2, 3 and 4 are wiring diagrams showing alternate forms of controlcircuits embodying the invention.

Referring to FIG. 1, an electric heating element 5 is shown controlledby a thermally responsive control device 6. The heating element may bearranged to heat a room or to actuate a temperature modifying system forthe room, for example. The control device 6 is located to respond tochanges in the air temperature in the room. 1

The control device 6 includes a suitable housing and circuit board, notshown, having terminal posts l2, l3 and 14. AC power is supplied by twopower lines L1, L2 connected to theterminals 12 and 13. The lines L1, L2are connected with a source of suitable current for energizing theheating element 5, such as V AC. One side of the heater element 5 isconnected with the terminal 14. The flow of energizing current for theheater element 5 is controlled by a semiconductor switch means 15connected between the terminal posts 13 and '14. The switch 15 ispreferably a Triac or its equivalent. Semiconductor switches of thistype are well known in the art and are not discussed in further detail.When a voltage of about 5 or 6 volts is impressed on the gate of theTriac 15 the Triac will conduct current until the voltage of the powercircuit swings to zero.

During the positive half cycles of the power supply, i.e., when terminal12 is positive relative to terminal 13, the gate voltage of the Triac 15is controlled by a gate controlled semiconductor or silicon rectifier l7(SCR). During the negative half cycle of the powersupply, i.e., when theterminal 13 is positive relative to the terminal 12, the gate of theTriac 15 is controlled by an SCR 18.

The gate of the SCR 17 is controlled by a temperature sensing triggeringbridge network 19 in response to the temperature of the air affected byoperation of the heater element 5. As explained more fully hereafter,when the room temperature is at a set level, the SCR 17 will not betriggered or fired and no current flows through the Triac 15 and theheating element 5. When the room temperature falls below the set level,the SCR 17 fires and renders the Triac 15 conductive for a substantialportion of the positive half cycle, thus energizing the heater element.The SCR 18 is also adapted to be triggered or fired duringthe negativehalf cycle by a slave circuit which is energized only in the event theTriac 15 was conductive during the preceding positive half cycle.

The temperature sensing bridge network 19 comprisesresistors 20, 21, 22and 23, a negative temperature coefficient thermistor 24 and apotentiometer 25. The bridge network is connected between the terminalsl2, 13 through a diode 26 and a voltage dropping resistor 27 so thatthenetwork is impressed with a half wave pulsating voltage when thevoltage at terminal 12 is positive relative to the terminal 13. Theresistance 20, potentiometer 25, resistance 23 and thermistor 24comprise a temperature sensitive bridge divider having a junction 30.The thermistor 24 is subjected to the temperature of the air affected byoperation of the heater element 5. The resistors 20, 21 are preferablyof wound wire having a high positive temperature coefficient. Theresistor 20 is in close heat exchange relation with a self-heatingresistor 32 connected in the power circuit in parallel with the heatingelement 5. The resistor 21 is subjected to the same ambient temperatureas is the resistor 20 so that the local ambient temperature effect onthese two resistors cancels the changes in resistances of thetemperature sensing bridge and the reference bridge caused bytemperature.

The level of voltage at junction 30 for a given temperature of thethermistor 24 and voltage impressed on the sensing network can be set oradjusted by the potentiometer 25. The resistors 21 and 22 comprise areference bridge divider having a junction 31. When the triggeringnetwork is subjected to a positive half cycle, the voltage at thejunctions 30 and 31 will increase as the power cycle swings from zeroand the temperature of the thermistor 24 and the setting of thepotentiometer 25 determines whether or not the rise in voltage at thejunction 30 will exceed or lag the rate of voltage rise at the junction31.

In accordance with the present invention, the temperature sensing bridgecircuit 19 is adapted to be shunted out of the power circuit during asubstantial portion of each positive cycle of the power supply by a gatecontrolled semiconductor (SCR) 33 in a bypass circuit connectingresistor 27 and the power circuit terminal 13. The gate of the SCR 33 iscontrolled in accordance with the voltage at junction 30 and istriggered to fire the SCR each time the voltage rises from zero to arelatively low level across the network and thereby shunt the networkout of the power circuit. Consequently the triggering network issubjected to but a fraction of the voltage and current of the powersource during operation of the control device 6.

The voltages at the bridge network junctions 30 and 31 are amplified bya differential amplifier and the output of the amplifier is impressed onthe gates of SCR l7 and SCR 33. The differential amplifier comprises twotransistors 34, 35 having their collectors connected in series with thedropping resistor 27 through a dropping resistor 37. The emitter of thetransistor 34 is connected with the power terminal 13 through aresistance 38 and the emitter of the transistor 35 is connected with thepower terminal 13 through a resistor 39. The resistors 38 and 39 areeffective to develop voltages at the output junctions 40 and 41,respectively, when the transistors 35 and 34 are conductive. Asthe'voltage applied across the bridge network 19 increases from zero oneor the other of the transistors 34 and 35 will be more forward biasedthen the other, depending upon the temperatures of 'the resistance 20,and the thermistor 24 and the setting of potentiometer25. Assuming thatthe potentiometer 25 was set for a given temperature and that thetemperature of the thermistor 24 satisfied the setting, the voltagelevel at 30 would lag that at 31 and transistor 35 would become moreconductive than transistor 34. SCR 33 would be triggered prior to SCR17' and the circuit for network 19 would then be shunted through SCR 33.This shunting action would dump the voltage at junction 41 so that SCR17 would not be triggered and the Triac 15 would remain nonconductive.

Should the temperature of the thermistor 24 fall below that desired, thevoltage level at the junction 31 will decrease over that at the junction30 during each positive half cycle of the power supply thereby causingthe transistor 34 to become more conductive than the transistor 35. Thisraises the voltage at junction 41 to a point to fire the SCR 17,triggering SCR 17 before the SCR 33 is triggered. SCR 17 therefore firesthe Triac 15 which becomes conductive and remains conductive for theremainder of the positive half cycle to energize the heater element 5.The voltage across the network 19 continues to rise after SCR 17 isfired and this voltage increase fires SCR 33 which establishes the shuntcircuit around the network circuit.

SCR 18 is fired at the beginning of the negative half cycle of the powersupply by discharging of a condenser 42 which is charged during thepreceding positive half cycle. One side of the condenser 42 is connectedwith power supply terminal 12 through a diode 43, which is arranged tobe forward biased during each positive swing of the supply voltage. Theother side of the condenser 42 is connected with the power supplyterminal 13 through the Triac 15. During any positive half cycle of thepower supply in which the Triac 15 is conducting, the condenser 42 willbe charged. During the following negative half cycle, the condenser 42discharges through a resistor 43 to the gate of SCR 18.- The SCR 18triggers the gate of the Triac 15 through a current limiting resistor 44and the Triac becomes conductive to energize the heater element 5 duringthe negative phase of the power cycle.

During energization of the heater element 5, the resistance element 32heats the bridge resistance 20 thereby tending to reduce the voltagelevel at the junction 30 relative to the voltage level at junction 31.Thus, the resistor 20 serves to effect cycling of the heating element bythe triggering circuit and minimizes temperature differential in theroom air.

By shunting the bridge network 19 early in each positive half cycle ofthe power supply, the voltage impressed on the bridge network neverexceeds a relatively low level and the current flow through the networkat the low voltage level is for a small portion of the total time ofoperation of the control circuit. This low current and voltage levelpermits the use of a minimum number of circuit components andparticularly permits the thermistor 24 to be relatively small and lightweight so that it responds rapidly to temperature changes. Furthermore,the limited flow of current in the bridge obviates self-heating and aresulting runaway condition in the thermistor which would otherwiseoccur if the bridge circuit were continuously subjected to line voltage.Other advantages of the invention are that the trigger circuits requireno power source other than the AC source thereby saving the cost ofcomponents while improving the time response and the reliability of thecircuits. I

Referring to FIG. 2, a second form of the invention is shown in whichthe heating element 5 is supplied with 115V AC current and its circuitis controlled by a temperature sensing device comprising a switchingmeans in the form of an'SCR 101 and a triggering circuit 102. The SCR101 and its triggering circuit 102 are supplied with unfilteredDC'current from the lines L1, L2 by a rectifying bridge comprised offour diodes 103, 104, 105 and 106. The triggering circuit for SCR 101 isconnected with the rectified current supply bridge through a droppingresistor 107. The triggering circuit includes a negative coefficientthermistor 110, a potentiometer 111 and resistors 112, 113 and 114. Theresistors 110, 111 and 112 form a temperature sensitive bridge dividerand the resistors 113 and 114 form a I reference bridge divider. Thegate of SCR 101 is connected to junction 115 between resistors 113 and114.

The triggering circuit 102 is adapted to be shunted out of circuit by anSCR 117 which is connected between lines L1 and L2 through diode 103,resistor 107 and diode 105. The gate of SCR 117 is connected withjunction 120 between potentiometer 111 and resistor 112.

In operation as the voltage of each DC pulse swings from zero, currentbegins to flow through the triggering circuit 102, developing potentialat the gates of both SCR 101 and 117. Should the voltage level for thegate of SCR 101 reach the firing level of the SCR before the voltage atthe gate for SCR 117 fires the latter, SCR 101 will be renderedconductive thereby energizing the heater element circuit. On the otherhand, if the voltage at the gate of SCR 117 reaches the firing potentialprior to the time the SCR 101 is fired, the SCR 117 will be renderedconductive and shunt the triggering circuit which dumps" the firingpotential for SCR 101' thereby preventing energizing of the heaterelement circuit except through resistor 107, which has a resistancesufficiently high that the element 5 will not "become heated.

Whether or not the potential at the gate of SCR 101 reaches its firinglevel ahead of that at the gate or SCR 117 depends upon the temperatureof the thermistor 110 and the setting of the potentiometer 111. In anyevent, the firing potential of both SCR 101 and SCR 117 are relativelylow, as in the order of 1 to 2 volts. Thus, the voltage to which thetriggering circuit is subjected is quite limited and the current flowoccurs only for a fraction of each current pulse.

Referring to FIG. 3, still another form of the invention is showninwhich the heater element 5 is connected in power lines L1, L2 throughtwo gate controlled semiconductors SCR 201 and SCR 202. SCR 201 and SCR202 are in parallel circuits and are arranged to conduct the respectivehalf waves of the full wave cycle of the AC current supply. The SCRs201, 202 are fired by a temperature sensitive triggering circuit 203.The triggering circuit is comprised of a thermistor 204, a potentiometer205 and a resistor 206. The triggering circuit 203 is connected withline L1 through a dropping resistor 207 and is connected with L2 througha dropping resistor 208. The triggering circuit 203 is adapted to beshunted by a circuit through an SCR 211 and alternatively to line L2through a diode 212 or to line Ll through a diode 213.

The gates of SCR 201 and SCR 202 are connected with resistor 206 throughdiodes 214 and215, respectively. The gate of SCR 211 is connected withthermistor 204.

In operation, when the current voltage of the AC power supply swingsfrom zero to a positive potential and the anode of SCR 201, currentflows through resistance 207 and thermistor 204 and applies a voltage onthe gate of SCR 211. Also, a voltage potential is developed on the gateof SCR 201 through diode 214. When the temperature of thermistor 204 isat the desired level, the voltage level on the gate of SCR 211 will beabove that on the gate of SCR 201 and SCR 211 will be triggered toconduct. The conduction of SCR 211 shunts the circuit 203 through thediode 212. the

In the event the potential on the gate of SCR 211 is lower than thatdeveloped at the gate of SCR 201, the latter will be triggered and willconduct the full current load for the heating element 5.

When the current flow reverses, a potential is applied to the anode ofSCR 202. Also, a potential is applied to the triggering circuit 203through resistor 208. A potential is applied to the gate of SCR 202through the diode 215. Likewise a potential is applied to the gate ofSCR 211. If the resistance of thermistor 204 is sufficiently low, SCR211 will be triggered before SCR 202 is triggered, thereby completing ashunt circuit around circuit 203 through the diode 213. If theresistance of thermistor 204 is relatively high due to relatively lowtemperature, the voltage level at the gate of SCR 202 will rise totriggeringlevel before SCR 211 is triggered. Thus, SCR 202 will conductthe full load current and energize the heating element.

The triggering voltage required for firing the SCRs 201, 202 and 211 arerelatively low so that the components of the triggering circuit 203 arenever subjected to load voltage and the duration of the energizingcurrent is relatively short for each half wave of current potential. I

Referring to FIG. 4, another form of the invention similar to FIG. 2 isshown in which the heater element 5 is controlled by a device 300 inseries with the load between the AC power supply lines L1, L2. Thedevice 300 includes an SCR 301 and a triggering circuit 302. The ACcurrent supplied by lines L1, L2 is rectified by a full wave rectifierbridge comprised-of diodes 303, 304, 305 and 306. The rectifier bridgeprovides unfiltered DC pulsations to the SCR 301 and to the triggeringcircuit 302. A dropping resistor 307 is interposed between therectifying bridge and the triggering circuit.

v The triggering circuit comprises a reference bridge dishunting ofcircuit 203 dumps the voltage at the gate of SCR 201 and that SCR is notfired nor rendered conductive. The resistance 207 prevents anysignificant current flow through the heater element 5. The potential atwhich SCR 21 1 is triggered is relatively low, such as l to 2 volts.

vider formed by resistance 310, potentiometer 311, resistor 312, and atemperature sensitive bridge divider formed by resistance 313 andthermistor 314. A differential amplifier responds to the voltagedifferentials at the divider circuit junctions 315 and 316. Thedifferential amplifier comprises two transistors 317 and 318, the basesof which are connected with the junctions 315 and 316, respectively. Thecollectors of the transistors are connected with resistor 307 through aresistor 320. The emitter of transistors 317 and 318 are connected withdiode 305 through resistors 321 and 322, respectively. The gate of SCR301 is connected with the junction 323 of the output of the transistor315 and the resistor 321.

A transistor 324 has its base connected with the junction 325 betweenthe output of transistor 316 and resistor 322. The collector of thetransistor 324 is connected with the base of the transistor 316 and itsemitter is connected with diode 305.

It is apparent that the network 300 is subjected to pulsating half wavesof the AC power source and that the degree of conduction through thetransistor 317 and 318 depends upon the voltages at junctions 315 and316, respectively. In the event that the temperature sensed by thethermistor 314 is at that desired according to the setting of thepotentiometer 311, the transistor 318 will be more conductive than thetransistor 317. Transistor 324 is turned on which then locks transistor318 in its full conductive condition by lowering the voltage at the baseof the latter transistor. The turning on of the transistors 318 and 324effectively shunts the bridge network comprising resistor 310,potentiometer 311, resistor 312, thermistor 314 and resistor 316. Theshunt circuit is through the resistor 320 which is many times theresistance of the heating element so that the heating element is notenergized. The transistors 318 and 324 become conductive at relativelylow voltages across the bridge network so that the flow of currentthrough the network is discontinued in the early portion of each swingof the voltage from zero.

ln the event that the temperature sensed by the thermistor 314 is belowthat for which the potentiometer 311 is set, the transistor 317 will bemore conductive than the transistor 318 and will fire SCR 301 prior tothe turning on of the shunting transistor 324. SCR 301 thereforeestablishes the energizing circuit for the heating element 5.

It will be understood that while the foregoing disclosures show thecontrol of a heating element to provide a given temperature, differentcondition sensitive bridge networks could be employed for producingvoltage levels for.controlling cooling equipment, equipment whoseoperation depends upon light or fluid sensing conditions, etc.

I claim:

1. Control means for an electric load including an AC power supply forsaid load providing voltages alternating in polarity relative to areference voltage, a first gate controlled switching means in circuitwith said power supply and said load and operative to complete anenergizing circuit for said load in response to a given triggeringvoltage applied to a gate thereof, a triggering circuit for the gate ofsaid switching means connected with said AC power supply and including acondition responsive resistance element, a second gate controlledswitching means connected with saidAC power supply and adapted to form ashunt circuit around said triggering circuit when said second gatecontrolled switching means is conductive and thereby substantially limitcurrent flow through said triggering circuit, and means to control thegate ,of said secondv gate controlled switching means to render saidsecond switching means conductive in response to a given change involtage value of said-AC supply from said reference voltage during eachalternation of said AC power supply, said triggering circuit adapted toeffect a triggering voltage for said first 'gate controlled switchingmeans in response to said changes in said AC voltage from said referencevoltage and prior to attainment of said given voltage, depending uponthe resistance of said condition responsive resistance element.

2. Control means as defined in claim 1 further characterized by saidfirst gate controlled switching means comprising a triac in series withsaid load, said triggering circuit comprising a differential amplifierand an SCR coupled to one output circuit of said differential amplifier,and said means to control the gate of said second gate controlledswitching means comprising another output circuit of said differentialamplifier.

3. Control means for an electric load including an AC power supply forsaid load providing voltages alternating in polarity relative to areference voltage, a first gate controlled switching means in circuitwith said power supply and said load and operative to complete anenergizing circuit for said load in response to a given triggeringvoltage applied to the gate thereof, triggering circuit means to controlthe gate of said switching means and connected with said AC powersupply, means for shunting said triggering circuit means comprising asecond gate controlled switching means for effecting current flowthrough said shunting means when said second gate controlled switchingmeans is conductive and thereby substantially limit current flow throughsaid triggering circuit, said triggering circuit means controlling thegate of said second gate controlled switching means and operable torender said second gate controlled switching means conductive, saidtriggering circuit means operative in response to a relatively smallchange in voltage value of said AC supply from said reference voltage torender one or the other of said switching means conductive during eachalternation of said AC power supply, and a condition responsiveimpedance element in said triggering circuit operable to vary thetriggering voltage at the gate of one of said gate controlled switchingmeans to selectively cause one of said switching means to conduct priorto conduction of the other of said switching means, depending upon theimpedance of said condition responsive impedance element.

4. Control means as defined in claim 3 further characterized by saidtriggering circuit means comprising first and second voltage dividercircuits, said first voltage divider circuit comprising said conditionresponsive impedance element and having an output which is dependent onthe resistance of said element, and said second voltage divider circuitcomprising substantially fixed impedances.

5. Control means as defined in claim 3 further characterized by saidcondition responsive impedance element operatable to vary the triggeringvoltage at the gate of said second gate controlled switching means.

6. Control means as defined in claim 4 further characterized by theoutput of said first voltage divider circuit operable to control thefiring of said second gate controlled switching means.

1. Control means for an electric load including an AC power supply forsaid load providing voltages alternating in polarity relative to areference voltage, a first gate controlled switching means in circuitwith said power supply and said load and operative to complete anenergizing circuit for said load in response to a given triggeringvoltage applied to a gate thereof, a triggering circuit for the gate ofsaid switching means connected with said AC power supply and including acondition responsive resistance element, a second gate controlledswitching means connected with said AC power supply and adapted to forma shunt circuit around said triggering circuit when said second gatecontrolled switching means is conductive and thereby substantially limitcurrent flow through said triggering circuit, and means To control thegate of said second gate controlled switching means to render saidsecond switching means conductive in response to a given change involtage value of said AC supply from said reference voltage during eachalternation of said AC power supply, said triggering circuit adapted toeffect a triggering voltage for said first gate controlled switchingmeans in response to said changes in said AC voltage from said referencevoltage and prior to attainment of said given voltage, depending uponthe resistance of said condition responsive resistance element. 2.Control means as defined in claim 1 further characterized by said firstgate controlled switching means comprising a triac in series with saidload, said triggering circuit comprising a differential amplifier and anSCR coupled to one output circuit of said differential amplifier, andsaid means to control the gate of said second gate controlled switchingmeans comprising another output circuit of said differential amplifier.3. Control means for an electric load including an AC power supply forsaid load providing voltages alternating in polarity relative to areference voltage, a first gate controlled switching means in circuitwith said power supply and said load and operative to complete anenergizing circuit for said load in response to a given triggeringvoltage applied to the gate thereof, triggering circuit means to controlthe gate of said switching means and connected with said AC powersupply, means for shunting said triggering circuit means comprising asecond gate controlled switching means for effecting current flowthrough said shunting means when said second gate controlled switchingmeans is conductive and thereby substantially limit current flow throughsaid triggering circuit, said triggering circuit means controlling thegate of said second gate controlled switching means and operable torender said second gate controlled switching means conductive, saidtriggering circuit means operative in response to a relatively smallchange in voltage value of said AC supply from said reference voltage torender one or the other of said switching means conductive during eachalternation of said AC power supply, and a condition responsiveimpedance element in said triggering circuit operable to vary thetriggering voltage at the gate of one of said gate controlled switchingmeans to selectively cause one of said switching means to conduct priorto conduction of the other of said switching means, depending upon theimpedance of said condition responsive impedance element.
 4. Controlmeans as defined in claim 3 further characterized by said triggeringcircuit means comprising first and second voltage divider circuits, saidfirst voltage divider circuit comprising said condition responsiveimpedance element and having an output which is dependent on theresistance of said element, and said second voltage divider circuitcomprising substantially fixed impedances.
 5. Control means as definedin claim 3 further characterized by said condition responsive impedanceelement operatable to vary the triggering voltage at the gate of saidsecond gate controlled switching means.
 6. Control means as defined inclaim 4 further characterized by the output of said first voltagedivider circuit operable to control the firing of said second gatecontrolled switching means.